Regulations are certainly a large obstacle in having a general purpose radio transmitter installed on an "open" consumer device. Scarcity of WLAN chipset documentation has already hinted at it.

However, real technical barriers exist. Encoding/decoding today's digital modulation schemes in pure software would not be economical using todays technology. Since we're talking about a mobile platform where low power consumption is top priority, it is safe to say that a software defined radio stays an impossibility for some years still.